The present invention relates generally to retransmission protocols for wireless communication systems and, more particularly, to the selection of retransmission parameters for hybrid automatic repeat request operations in wireless communication systems.
The High Speed Downlink Packet Access for Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE) networks use Hybrid Automatic Repeat Request (HARQ) at the physical layer to mitigate errors that occur during transmission of data. In HARQ, error detection bits or check bits are added to information bits to be transmitted. The information bits with the added error detection bits are then coded using a forward error correction code and transmitted to the receiver. The receiver decodes the received bits and uses the error detection bits to check for uncorrected errors. If the received data block is not correctly decoded, the receiver can request a retransmission by sending a negative acknowledgement (NACK) to the transmitter over a reverse control channel, For type-I HARQ, the retransmission comprises the same bits sent in the initial transmission. For type-II HARQ, sometimes referred to as HARQ with incremental redundancy, the coded bits for each retransmission differ from the coded bits sent in the initial transmission.
In LTE systems, the information bits with added error detection bits are coded by a turbo coder to generate a block of coded bits. The coded bits in the code block are interleaved and input to a circular buffer. To support HARQ operations, four redundancy versions (RVs) are defined. Each redundancy version is associated with a different RV starting point in the circular buffer. During an initial transmission or retransmission, bits are read out of the circular buffer beginning with the RV starting point corresponding with the selected redundancy version. The number of bits read out determines the effective code rate. For purposes of this application, the redundancy versions are designated as RV0, RV1, RV2, and RV3, and it is assumed that RV0, which contains most of the systematic bits, is used for the initial transmission in HARQ operations.
Because of the simplicity and regularity of the circular buffer employed by the channel coder in LTE systems, it is relatively easy to select the redundancy version for a retransmission that provides the greatest incremental redundancy gains. When RV0 is transmitted during an initial transmission, RV2 provides the greatest incremental redundancy gains for quadrature phase shift keying (QPSK) and other modulation schemes in which the bit reliability is the same for all bits in a modulation symbol. For this reason, the LTE standard currently specifies a fixed retransmission strategy for the non-adaptive HARQ mode of PUSCH wherein RV2 is specified for retransmission when RV0 is used for the initial transmission.
Optimization of the HARQ operation is more difficult, however, for higher order modulation schemes, such as 16QAM and 64QAM, where the bits in a QAM symbol fall into different reliability classes. In this case, choosing the redundancy version for a retransmission that provides the greatest incremental redundancy gain may not necessarily result in the best performance. For example, it was observed in simulations that RV3 provides greater improvement in performance over RV2 when RV0 is transmitted in the initial transmission for some selected code rates. Accordingly, there is a need for further improvement in the optimization of retransmission settings for HARQ operations.